Silver layers on mirrors are extremely sensitive to the presence and corrosive action of airborne contaminants, e.g., salt particles typical in coastal regions, or H.sub.2 S, NH.sub.3 or acidic contaminants which are always present in domestic or urban environments. These contaminants, in the presence of moisture, are able to promote oxidative processes which occur according to the reaction Ag-e.fwdarw.Ag.sup.+ and result in the corrosive disintegration of the mirror's reflective layer. Specialized corrosion preventive coating systems, known as "mirror backing" coatings are applied in order to extend a silver mirror's service life.
It is known that non-pigmented protective coatings formulated without a corrosion inhibitor pigment component, exhibit limited anti-corrosion protection and consequently do not provide long term service life. Therefore, corrosion inhibitor pigments are critically important functional components of mirror coating systems and basically determine the useful service life of the protected silver surface.
It is known in the prior art to use various lead salts, such as lead cyanamide as corrosion retardant pigment components of mirror backing formulations. The specific corrosion retardant activity displayed by these compounds on Ag surfaces is attributed to the presence of lead species and perhaps to their S.sup.2- scavenging capacity. Due to the excellent performance of mirror backing formulations containing lead compounds, such systems have been widely used for decades by the mirror manufacturing industry. Efforts to develop lead-free alternatives having anticorrosive activity for silver have been spurred by environmental concerns.
U.S. Pat. No. 4,707,405/1987 (Evans et al.) discloses the use of metal derivatives of hydrogen cyanamide other than lead cyanamide as corrosion retardant pigment components in mirror backing protective coatings. However that patent limits the concept exclusively to cyanamides formed by Group IIA and IIB elements such as magnesium, calcium and zinc.
There are several chemical and physical properties which a pigment grade product must possess in order to function as a component of a paint formulation and/or mirror backing protective coating. Among the required characteristics are limited water solubility, moderate alkalinity and compatibility with resins and solvents used in such formulations. Specifically the capacity to interact with, and provide inhibitive activity against substrate specific electro-chemical oxidant processes, which promote corrosive decomposition of Ag surfaces, are the most important, and are determined by the pigment's chemical composition and structure. In this sense the disclosures of the above identified U.S. Patent leave a need for further improved pigment systems for use in mirror backing formulations.
Zinc cyanamide is known for its valuable pigmentary proprieties and its applicability as anticorrosive pigment in primer formulations recommended i.e. for steel surface protection. In my U.S. Pat. No. 5,176,894 issued Jan. 5, 1993 I disclosed a pigment grade zinc cyanamide which meets the quality requirements for a mirror backing protective coating, i.e., high assay and absence of soluble salts.
It is well known in the chemical literature that hydrogen cyanamide, a di-basic acid, forms neutral, as well as basic and acidic salt derivatives with numerous metals, inclusive of the Group IA, IIA elements and some metals of the first, second and third transition series, among others with Co.sup.2+ and Ni.sup.2+.
It is important to note, however, that Ni.sup.2+ and Co.sup.2+ are the only species known to form the bis-hydrogen cyanamide structures symbolized by M (HNCN).sub.2 M .sup.2+ =Ni.sup.2+, Co.sup.2+) where the M /NCN stoichiometrical ratio is 1:2.
Considering that the .dbd.N--C.tbd.N moiety of any cyanamide compound, due to its characteristic structure, is likely to generate the electrochemically active inhibitor species (by interacting with moisture in situ in the protective coating) accountable for substrate specific corrosion preventive activity on silver, it becomes evident that such derivatives characterized by "bis" structure should be preferably employed as inhibitor pigments. In support of this observation it will be noted that the theoretical value in weight % of the "NCN" content for ZnNCN is 37.9 compared to 58.1 for Ni or Co bis-hydrogen cyanamide.
Bernard et al. [Compt. Rend. Ser. C 262(3), 282-4 (1966) report that Ni.sup.2+ and Co.sup.2+ species form cyanamide derivatives of bis-hydrogen cyanamide structure and disclose a relevant preparation procedure based on the precipitation reaction involving hexamine-nickelate or cobaltate species and H.sub.2 NCN in ammoniacal medium, as follows: EQU M SO.sub.4 +6NH.sub.4 OH.fwdarw.[M (NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +6H.sub.2 O 1. EQU [M (NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +2H.sub.2 NCN.revreaction.M (HNCN).sub.2 H.sub.2 O.dwnarw.+(NH.sub.4).sub.2 SO.sub.4 +4NH.sub.4 OH 2. EQU where M =Co.sup.2+ or Ni.sup.2+
Typically the procedure is carried out by introducing H.sub.2 NCN into ammoniacal solution of hexamine-nickelate or -cobaltate and by subsequent agitation of the system for 18 hours at pH=7.5. A similar procedure is disclosed, specified exclusively for Ni.sup.2+ cyanamide in Example #4 of Japanese Patent Nr. SHO 29-8020/12.07.54. The process is performed in one hour by simultaneously introducing nickel sulfate solution and ammonia gas into hydrogen cyanamide solution, at 25.degree.-30.degree. C. and by keeping the pH of the system at 7.5 to 8.5.
The NiSO.sub.4 /H.sub.2 NCN=1:1 molar ratio, recommended, quite surprisingly, by this Japanese patent, fails to consider the bis-hydrogen cyanamide structure of the intended product and represents a basic stoichiometrical error (See reaction 2.), which results in particularly low yield (of about 53% in Ni(HNCN).sub.2 H.sub.2 O) based on the disclosed value, and the correspondingly high amount (practically 50%) of unconverted NiSO.sub.4 dissolved and lost in the process water.
As expected, the disclosed value of the obtained product's nitrogen content (33.2%N) is consistent with bis-hydrogen cyanamide composition, however of a relatively poor quality, which is a direct consequence, as well, of the employed inadequate NiSO.sub.4 /H.sub.2 NCN molar ratio. For the same reason under the final conditions of the process (absence of H.sub.2 NCN, high Ni.sup.2+ concentration, Ph.about.8.0) basic divalent nickel salts also precipitate which, by subsequent dehydration are converted into dark colored, inactive inclusions in the final products.
In addition to the above-exemplified shortcomings, there are inherent limitations of the manufacturing procedures known by the prior art, all specifically related to the precipitation reaction involving hexamine-nickelate or -cobaltate and H.sub.2 NCN, respectively to the employment of ammonia as pH control reagent. Beside the inconvenience caused by the volatility of NH.sub.3 at the recommended pH value, which require the employment of protective technologies, the following limitations are observed:
1. Reaction 2, a typical process of precipitation involving dissolved hexamine nickelate or cobaltate species, reaches an equilibrium which obstructs the completion of the direct reaction (basically the formation of the product by precipitation) to the desirable extent, even at substantial stoichiometrical excesses of H.sub.2 NCN. This undesirable characteristic of the reaction system is a direct consequence of the presence of ammonium salts, soluble by-products formed according to reaction 2, capable to prevent the complete precipitation of Ni.sup.2+ or Co.sup.2+ species as cyanamides. Thus the yield of the process is substantially reduced and the resulting process water (mother liquor and wash water) contains large amounts of irrecuperable Ni.sup.2+ species as well as undesirable ammonium salts.
2. As the complete removal of the soluble by-products (usually accomplished by extensive washing) is the critical phase of any corrosion retardant pigment manufacturing process which essentially determines the quality of the final product, processes that use ammonia for pH control, for aforementioned reasons, result in large amounts of non-recyclable, environmentally hazardous process water with high Ni.sup.2+ or Co.sup.2+ contents.